full post versus excerpt are 2 totally different things. by default, thematic only shows the featured image in the excerpt mode. it doesn't automatically show up on the single post. of course you can add it in by filtering thematic_post.

if you are using a program like notepad++ (which you should, i recommend it as a kick ass text editor) then you can just search content-extensions.php for 'thematic_post'. you'll find the filter is in the thematic_content() function. i originally thought that the filter name was the same as the function, but this isn't the case. the filter name is what is seen in the apply_filters(). here's the source: